This invention relates to electronically tuned radio transceivers and particularly to transceivers using a freguency synthesizer with coded channel frequency information stored in a memory.
Handheld radio transceivers have existed for some time, the early models using electromechanical tuning devices to select the frequency of operation. Electronically tuned receivers were subsequently developed which incorporated multiple local oscillator circuits each having a crystal of a desired frequency whereby the operator could tune to a selected frequency by switching in the corresponding crystal. The phase-locked loop freguency synthesizer has afforded a precision of tuning not previously obtainable due to the vulnerability of electromechanical devices to thermal drift and other causes of error. The use of completely electronic devices including, in many cases, large-scale integrated circuits, has brought about greater reliability, smaller size and lower cost.
Current transceivers have multiple channel operation and thereby provide some measure of selectivity of communication. However, the number of communication channels available is necessarily limited by the number of frequencies alotted for use by the Federal Communications Commission (FCC).
U.S. Pat. No. 4,245,504 to Lewis et al discloses a transceiver with a frequency synthesizer controlled by channel frequency codes contained in a programmable read only memory (PROM), with a channel switch for addressing the memory and thereby selecting the channel frequency. This apparatus is not capable of selective communication on a single carrier frequency. Any transmission is received by all users who are tuned to the frequency of the transmission whether it is intended for them or not.
Selective transmission of intelligence is possible on a single carrier frequency by using a tone control circuit having a frequency-controllable filter, as shown, for example, in U.S. Pat. No. 3,597.690 to Wycoff. That apparatus uses switches connected to tapped inductors to detect selected tone frequencies. However, the number of available tone frequencies is severely limited, as a practical matter, by the number of taps possible on a tapped inductor. Moreover, the Wycoff apparatus has no provision for more than one carrier frequency. nor does it provide for changing to a different tone frequency in conjunction with a change of carrier frequency.
A two-way radio using a frequency synthesizer and storing carrier and tone frequency data in an electrically alterable read only memory (EAROM) is manufactured by the Wilson Division of Regency Electronics, Inc. and designated model WH 2510. Separate channels of this radio may be programmed for different carrier and tone frequencies with each channel having different transmit and receive mode carrier frequencies and a common tone frequency for both transmission and reception.
Transceivers such as those described above have limitations where, for example, they are used for multiple communication purposes on the premises of factories, construction sites, warehouses and other facilities. Because such communications networks typically employ more than two radios on one frequency, users are frequently disturbed by unwanted calls. Selective communication is not readily available, however, without assigning separate carrier frequencies to individual personnel or resorting to more complex and expensive selective signalling techniques. Except for low-power industrial frequencies, business and industrial users are not able to assign separate carrier frequencies because such users are limited by the FCC to transmission on one carrier frequency. Although multiple frequency assignments can be made on certain frequencies set aside by the FCC for low-power business and industrial communications, this technique increases the likelihood of interference and is impractical in repeater applications. Moreover, programming of tone frequencies in radios such as the Wilson radio only provides a limited degree of selective communication.
Another limitation of radios such as the Wilson radio concerns their use with repeaters. Repeaters, which transmit and receive on separate carrier frequencies, are susceptible to unauthorized utilization. A repeater's transmit carrier frequency is readily determined with the aid of a scanning receiver, and since the receive and transmit frequencies of a repeater are commonly separated by a fixed difference frequency, the repeater's receive frequency is easily calculable. The frequency of the tone transmitted by the repeater can also be measured with relatively simple equipment. With this information, anyone desiring to do so can activate and use the repeater without authorization. Unauthorized access can have costly consequences inasmuch as many repeaters function as interconnections to phone lines or computers. Such unauthorized access can be made more difficult by using independent tone frequencies for transmission and reception. However, transceivers which have a common tone freguency for transmission and reception on given channels would not be operable in such a communication system.